This invention relates to UHF, high power broadcast antennas systems and particularly to such a system for transmitting TV broadcast channels 19 to 70, in which the transmission line run up a tower from the transmitter to the radiating antenna at the top of the tower includes a substantial length of circular waveguide.
Broadcast antennas for television are frequently located on top of high towers that are constructed to offer minimum wind resistance. The transmitter is located on the ground near the tower and the radiating antenna is located at the top of the tower. A transmission line from the transmitter to the antenna is carried by the tower, either inside or the outside of the tower structure and is attached thereto by hardware that includes hangers, ties, etc. The cost of the tower, the transmission line, its hardware, elbows, transitions, transformers, barriers, hangers ties, etc., constitute just about the total cost of the antenna system. That cost, the cost of maintenance and the cost of operation are all important considerations. The cost of operation is directly related to the efficiency of the transmission line from the transmitter to the antenna (everything else being equal).
Recently, UHF, high power broadcast TV antennas for channels 21 through 70 that transmit 100 kilowatts (kw) and more, from an antenna on a tower that may be 2,000 feet high, have used circular instead of rectangular waveguide or coaxial transmission line to conduct the UHF power up the tower to a radiating antenna on top of the tower. The advantages of circular waveguide over coaxial transmission line are greater efficiency, higher power carrying capability and lower cost. The amount of electric power input to the system is very sensitive to the efficiency of the transmission line. An increase of 10% in the efficiency of the transmission line can reduce the power requirement by 30%.
Another advantage of waveguide over coaxial is that waveguides are high pass filters and so no energy will propagate in a waveguide below a UHF cut off frequency, whereas coaxial line will conduct from very low to very high frequencies. Also, coaxial line cost more per foot than waveguide and is more costly to operate than waveguide, because the equivalent coaxial line has greater attenuation. These cost and attenuation disadvantages of coaxial line relative to waveguide increase (get worse) as the tower height is increased.
The advantages of circular over rectangular waveguide are: lower wind load, greater efficiency, lower installation costs, no torsional twists due to wind and no rotation due to manufacturing twist. The disadvantage of circular waveguide is that: unless the circularity of the circular waveguide along its entire length is maintained within very precise tolerances, the desired polarization mode of waves launched into the circular waveguide in the fundamental TE.sub.11 propagation mode is accompanied by an undesired cross (transverse) polarization mode in the same TE.sub.11 propagation mode and since the efficiency of the coupling from the circular waveguide to the antenna at the top of the tower is highly sensitive to the direction of polarization (herein called the polarization mode) of this fundamental propagation TE.sub.11 mode, the efficiency of coupling from the circular waveguide to the antenna suffers.
The cross polarization mode in a circular waveguide arises as follows. The TE.sub.11 mode in circular waveguide is linearly polarized, but unlike rectangular waveguide, there can be the same propagation mode polarized transverse (cross) thereto. Theoretically, the two cross polarized modes in the circular waveguide are entirely independent of each other when the circularity of the waveguide is perfect. However, since structural circularity is not perfect and some ellipticity of the waveguide occurs, it is possible to create an elliptically polarized wave that will cause energy to be coupled from the desired to the undesired cross polarization mode. This coupling is most severe when the desired polarization mode direction is at forty five degrees to the major axis of the structural ellipse. The cross polarization can be minimized by maintaining close tolerances of circularity of the waveguide (within a few thousandths of an inch).
Non-circularity of a circular waveguide reduces efficiency. The undesired cross polarized mode wave gets trapped between transition sections that couple the circular waveguide to other transmission lines, such as rectangular waveguides at the top and bottom of the tower, resulting in a high voltage standing wave ratio (VSWR) therebetween. To avoid this, the circular geometry of the circular waveguide would have to be within a tolerance of a few thousandth of an inch for the trapped wave to be negligible. This problem of non-circularity is overcome using a technique taught in my co-pending U.S. patent application Ser. No. 449,734, filed Dec. 14, 1982 and entitled: UHF Broadcast Antenna On A Tower With Circular Waveguides Carrying RF Energy Up The Tower To The Antenna. That technique consists of causing a complementary elliptical polarization in the circular waveguide at the bottom of the tower in compensation for the undesired cross polarization mode caused by imperfections throughout the length of the circular waveguide run up the tower. The complementary, the desired and the undesired modes combine so that at the output end of the circular waveguide run there results a pure linear polarized wave that can be efficiently coupled to the radiating antenna at the top of the tower.
The many imperfections that may occur in a two thousand foot run of circular waveguide up a tower are of several kinds. There are those that produce discontinuity such as: offsets, where two sections of circular waveguide connect together; tilt, where the axis of the two sections define a slight angle; or even diameter changes from section to section; and non-circularity of the waveguide. All of these discontinuities tend to create new wave propagation modes. However, by careful design of the overall system in the selection of the circular waveguide diameter for the operating frequency and using special transitions between the circular waveguide and the rectangular waveguide feed at the bottom of the tower and also at the top between the circular waveguide and the antenna, the new modes are not propagated. Such special transitions are described in my co-pending U.S. patent application Ser. No. 484,220, filed Apr. 12, 1983, entitled: Transition Between Rectangular And Relatively Large Circular Waveguide For A UHF Broadcast Antenna. As taught therein, the new propagation modes created by the discontinuities will be well below the cutoff frequency of the circular waveguide and so will be down sixty dB and more.
As described in my first mentioned co-pending U.S. patent application Ser. No. 449,734, the complementary elliptical polarization is created at the bottom of the tower by an attachment to the circular waveguide called an ellipse generator that distorts the waveguide crosswise dimensions and is adjustable as to the direction and force of distortion. Following that, the circular waveguide to rectangular waveguide transition sections at the output end of the circular waveguide run (see the above mentioned patent application) are rotated as a unit so that the pure linear TE.sub.11 polarization wave produced at that end of the circular waveguide is aligned with the electric (E) field of the TE.sub.10 mode of the rectangular waveguide end of the transitions. However, where the transmission line at the top of the tower that feeds the antenna is rectangular waveguide, it often occurs that the rectangular waveguide end of the transition is not aligned with the rectangular waveguide feed line and so another adjustment must be made to align them. It is one object of the present invention to provide a method and means in such a circular waveguide transmission line for bringing the pure linear polarized wave from the circular waveguide and the transmission line at the top of the tower that feeds the antenna, into alignment with each other for efficient transmission.
Having adjusted the ellipse generator, at the bottom of the tower and the circular to rectangular waveguide transition at the top of the tower, as described above, and then aligned the rectangular waveguide end of the transition with the antenna feed line, the above object is accomplished, and all major imperfections in the system likely to increase VSWR and cause ghost signals are corrected and/or compensated for. However, this can be upset. It can be upset by changes in ambient temperature and wind gusts which cause dimensional changes in the circular waveguide. Other causes of upset can arise shortly after the erection of the tower and the vertical run, when the circular waveguide transmission line settles on the hangers that connect it to the tower and mechanical stresses are introduced that were not there before. It is another object of the present invention to provide a method and means of preventing the undesired cross polarization mode that arises at any time from energizing the antenna at the top of the tower.